The invention relates to a flat cable and a process for producing the same, and more particularly to a flat cable, which can eliminate the need to increase the width dimension without detriment to electric characteristics and further has a stable structure, and a process for producing the same.
A flat cable generally comprises: linear portions each comprising a plurality of insulated conductors juxtaposed to one another, the insulated conductors each comprising a conductor covered with an insulative layer; and twisted pair portions provided alternately with the linear portions, the twisted pair portions each comprising a plurality of twisted pairs juxtaposed to each other or one another, the twisted pairs each being composed of two insulated conductors which extend from the linear portions and are twisted together.
In flat cables of this type, the construction of the linear portion is an important element. Improper construction of this portion, for example, adversely affects the connection of conductors of the insulated conductors in the linear portion to a connector, making it impossible for the flat cable to have contemplated functions.
In a first conventional flat cable, the linear portion has such a construction that a plurality of insulated conductors each comprising a conductor covered with an insulative layer are provided side by side while leaving a spacing between adjacent insulated conductors, and the plurality of insulated conductors are sandwiched between upper and lower resin tapes in such a manner that the upper and lower resin tapes are fused to each other in a portion between adjacent insulated conductors.
In a second conventional flat cable, the linear portion comprises: a plurality of insulated conductors with adjacent insulated conductors in their insulative layers being fused to each other to constitute an integral insulative layer having crest portions and root portions; and a resin tape which is provided on one surface of the integral insulative layer so that it is spottily adhered to the integral insulative layer in its crest portions.
Flat cables having the above constructions, wherein the linear portions have been flattened by the resin tape, have been extensively used for internal wiring in electronic equipment, such as personal computers.
The conventional flat cables having the above constructions, however, have the following drawbacks. In the linear portions of the first conventional flat cable, the resin tape is provided between adjacent insulated conductors. This results in increased width dimension of the flat cable. In the case of flat cables, in many cases, the width dimension is regulated. The construction of the first conventional flat cable in its linear portions makes it difficult to follow the regulation value. Reducing the diameter of the insulated conductors is considered effective for overcoming this difficulty. This, however, leads to deteriorated electric characteristics of the flat cable. Further, regarding the dimensional accuracy, in the linear portions of the first conventional flat cable, the accuracy of the pitch between insulated conductors is low due to the presence of the resin tape between adjacent insulated conductors. This is likely to cause a failure of fitting at the time of pressure contact connection.
On the other hand, according to the construction of the linear portions in the second flat cable, there is no need to widen the portion between adjacent insulated conductors, and the accuracy of pitch between insulated conductors is high. In the linear portions of the second conventional flat cable, however, since the resin tape is spottily adhered to the insulative layer, there is a fear of the resin tape being separated from the insulative layer. Further, this construction has an additional problem that, at the time of simultaneous connection of conductors of the insulated conductors to a connector, it is difficult to insert pins of the connector, leading to poor fitting between the cable and the connector.
Next, a general conventional production process of flat cables will be explained. A predetermined number of insulated conductor pairs are first provided. Each of the insulated conductor pair is composed of two insulated conductors each comprising a conductor covered with an insulative layer, and has linear portions and twisted pair portions provided alternately with the linear portions at predetermined spacing. In the linear portions, the two insulated conductors are juxtaposed to each other, while in the twisted pair portions, the two insulated conductors are twisted together. The predetermined number of insulated conductor pairs are placed side by side and, in this state, are fed from a bobbin (not shown) into a mold. The mold is split into two parts, openable upper mold and lower mold. The upper and lower molds have grooves the number of which is such as will be able to receive all the insulated conductors in the linear portions. In the grooves, the insulated conductors are placed in such a positional relationship that adjacent conductors come into contact with each other. In this state, upon application of heat from the mold, the adjacent insulated conductors in their insulative layers are fused to each other to form a belt comprising insulated conductors connected to one another, thereby providing a linear portion wherein the surface of the integrated insulative layer has crest portions and root portions as viewed in the cross section of the linear portion.
The fusion of the insulated conductors to one another is carried out on a batch basis rather than a continuous basis. After the formation of the linear portion, the mold is opened. In this state, the twisted portion is passed through the mold. A next linear portion is then fed into the mold, and fusion of the insulated conductors to one another is again carried out. The above procedure is then repeated.
After the insulated conductor pairs in their linear portion are integrated with one another in the mold, they emerge from the mold, are passed through a take-up unit, and, together with a resin tape, are fed into between heating rollers to fuse the resin tape to one surface thereof. Thus, a predetermined flat cable is obtained wherein, in the linear portions, the resin tape has been spottily adhered onto the integral insulative layer in its crest portions.
The flat cable, of which the flatness of the linear portions is kept by the resin tape, enables simultaneous connection of conductors in the linear portion to a connector and hence has been extensively used in various types of electronic equipment including personal computers, and a further increase in demand thereof is expected.
In the conventional process of a flat cable, the adhesion of the resin tape to the integral insulative layer is intermittent. That is, the resin tape is adhered to only the crest portions of the integral insulative layer. This leads to a fear of the resin tape being separated from the insulative layer. This further poses a problem that, at the time of simultaneous connection of conductors in the linear portion to a connector, the resin tape in its portion floating above the root portions of the integral insulative layer inhibits fitting between the linear portion and the connector. The above problems are attributable to fusion using heating rollers, and hence are unavoidable without the elimination of reliance of the fusion upon the heating rollers.
Accordingly, it is an object of the invention to provide a flat cable that can eliminate the need to increase the width dimension without detriment to electric characteristics, has high accuracy of pitch between insulated conductors, and has a stable structure having no fear of the resin tape being separated from the insulative layer.
It is another aspect of the invention to provide a process for producing a flat cable that enables the resin tape to be fused to the resin layer so as to conform to the shape of the surface of the insulative layer without spot fusion and, in addition, so as to prevent the inhibition of the resin tape against simultaneous connection of conductors in the linear portion of the flat cable to a connector.
According to the first feature of the invention, a flat cable comprises: a plurality of insulated conductors each comprising a conductor covered with an insulative layer, said plurality of insulated conductors being juxtaposed to one another with adjacent insulated conductors in their insulative layers being adhered to each other, to provide an integral insulative layer having crest portions and root portions as viewed in the cross section of the juxtaposed insulated conductors; and a resin tape applied to the surface of the integral insulative layer so as to conform to the shape of the surface of the integral insulative layer having the crest and root portions.
The application of the above constitution of the invention is not limited to linear portions in a flat cable comprising linear portions and twisted pair portions provided alternately with the linear portions. For example, the constitution of the invention can be applied to various forms of flat cables including those which have, instead of twisted pair portions, roller screen-like portions each composed of insulated conductors merely juxtaposed to one another without adhesion of adjacent insulated conductors, and those free from twisted pair portions or roller screen-like portions.
The resin tape may be applied onto both surfaces or one surface of the integral insulative layer in the juxtaposed insulated conductors. The application of the resin tape onto both surfaces of the integral insulative layer in the juxtaposed insulated conductors may be carried out according to the following two preferred embodimetns. One of them is such that a predetermined part of the insulated conductor located at each end of the juxtaposed plurality of insulated conductors remains exposed without application of the resin tape thereto. The other preferred embodiment is such that the resin tape is extended by a predetermined length from the side of the insulated conductor located at each end of the plurality of insulated conductors juxtaposed to one another and the extended portion of the resin tape applied onto the upper surface of the integral insulative layer in the juxtaposed insulated conductors is adhered to the extended portion of the resin tape applied onto the lower surface of the integral insulative layer in the juxtaposed insulated conductors.
The former embodiment is suitable when the resin tape in its portion extended from each of the outermost located insulated conductors adversely affects the connection of the flat cable to a connector. On the other hand, the latter embodiment is suitable when the flat cable is used in applications where all the insulated conductors should be protected by the resin tape.
In many cases, heat fusion is utilized in adhesion between adjacent insulative layers in the insulated conductors, in the application of the resin tape to the insulative layers, or in adhesion between extended portions in the resin tape.
When the material for the insulative layer is different from the material for the resin tape and it is difficult to fuse the insulative layer to the resin tape, preferably, an adhesive layer having good fusion to the insulative layer is coated on the surface of the resin tape.
Specifically, preferred is a combination of an insulative layer formed of a vinyl chloride polymer, such as polyvinyl chloride, or an ethylene polymer, such as polyethylene, with a polyester tape coated with an adhesive layer of a vinyl chloride polymer or an ethylene polymer.
According to the second feature of the invention, a process for producing a flat cable, comprises the steps of: feeding a plurality of insulated conductors, juxtaposed to one another, together with a resin tape into a mold, the plurality of insulated conductors each comprising a conductor covered with an insulative layer, said mold comprising a combination of openable upper and lower molds, the upper and lower molds each having in its inner surface a plurality of grooves for accommodating therein the plurality of insulated conductors, the mold being constructed so that, upon accommodation of the plurality of insulated conductors respectively in the plurality of grooves in the mold, adjacent insulated conductors come into contact with each other;
after the accommodation of the plurality of insulated conductors and the resin tape in the plurality of grooves of the mold, closing the upper and lower molds to confine the plurality of insulated conductors and the resin tape within the plurality of the grooves; and
applying heat to fuse adjacent insulated conductors in their insulative layers to each other to form an integral insulative layer having on its surface crest portions and root portions as viewed in the cross section of the insulated conductors, and to fuse the resin tape to the integral insulative layer so as to conform to the shape of the surface of the integral insulative layer including crest portions and root portions.
The production process of the invention is not limited to flat cables having linear portions and twisted pair portions provided alternately with the linear portions. For example, the production process of the invention can be applied to many forms of flat cables including those which have, instead of twisted pair portions, roller screen-like portions each composed of insulated conductors merely juxtaposed to one another without adhesion of adjacent insulated conductors, and those which are free from twisted pair portions and roller screen-like portions and are entirely constituted by the linear portion.
The resin tape maybe applied onto both surfaces or one surface of the integral insulative layer in the juxtaposed insulated conductors.
In most cases, the material constituting the insulative layers of the insulated conductors used is the same as or similar in molecular structure to the material constituting the resin tape used from the viewpoint of fusion between the insulative layers and the resin tape. In some cases, however, the material constituting the insulative layers of the insulated conductors used is utterly different from the material constituting the resin tape used.
Specifically, for the insulative layer, importance is attached to electric characteristics, while for the resin tape, importance is attached to mechanical properties. An example of a combination, of the insulative layer with the resin tape, capable of providing both good electric characteristics and good mechanical properties is such that the insulative layer is formed of a vinyl chloride polymer, such as polyvinyl chloride, or an ethylene polymer, such as polyethylene, while the resin tape is a polyester tape coated with an adhesive layer of a vinyl chloride polymer or an ethylene polymer.
In this combination, since the polyester tape possesses excellent mechanical properties, a flat cable having a stable structure can be produced.
When the production of a flat cable having linear portions and twisted pair portions is contemplated, the resin tape is fused to the twisted pair portion by means of a heating roller after or before the completion of fusion between the insulative layers of the insulated conductors in the linear portion and fusion between the resin tape and the integral insulative layer formed as a result of the fusion between the insulative layers of the insulated conductors in the linear portion.
According to one embodiment of the invention, in the production of a flat cable, one or a few dummy linear materials may be disposed on both ends of the plurality of insulated conductors juxtaposed to one another.
The linear material may be made of a material not fused to the insulative layers in the insulated conductors and the resin tape. Specific examples thereof include electric wires covered with a fluororesin which is excellent in this property.
The linear material functions to prevent the flow of the resin from the product toward the widthwise direction during the production of the flat cable. In this case, additional grooves for the linear material are provided in the upper and lower molds. After the completion of the flat cable, the linear material is removed from the side of the cable.